(ci) Clean-up Travis-CI
(ci) Clean-up Travis-CI

Upstream changes broke Travis-CI, the fix is documented here: https://travis-ci.community/t/clang-10-was-recently-broken-on-linux/11527/2

Of course, I can't leave well enough alone, and this fix looks pretty ugly. If the issue is just Valgrind, then limit the workaround to just that job.

acgetchell/CDT-plusplus,acgetchell/CDT-plusplus,acgetchell/CDT-plusplus